The 3rd Generation Partnership Project (3GPP) Long Term Evolution (LTE) of the Universal Mobile Telecommunications Systems (UMTS) is expected to permit up to 4 antenna ports to be defined for multi-antenna base station transmissions and to permit using 1, 2 or 4 antenna ports for selected physical channel transmissions. The Physical Broadcast Channel (PBCH) may be transmitted using all three of these latter antenna port configurations. Since the base station does not explicitly signal the antenna configuration via the synchronization channel, the user equipment (UE) is required to decode the PBCH without the assistance of base station antenna configuration information acquired during an earlier phase of the initial network access procedure. In particular, the PBCH-borne Master Information Block is transmitted as a convolutionally encoded codeword with an inner cyclic redundancy check (CRC), but it is possible even at high signal-to-noise ratios for the UE to fail to identify the number of antennas present solely by inspection of the common reference symbols (RS). Similarly, it is possible for the UE to incorrectly identify the base station antenna configuration when hypothesis-testing the transmit diversity scheme associated with each permitted antenna configuration in combination with a PBCH CRC testing. For example, when transmitting using the specified 2 antenna transmit diversity scheme of space-frequency block coding (SFBC), a UE can correctly decode the PBCH codeword when hypothesizing (incorrectly) 1 antenna transmission.
3GPP R1-073970 discloses several possible approaches to communicating base station antenna configuration information for corresponding PBCH transmissions. In one approach, the mapping of a PBCH codeword to OFDM symbols and sub-carriers (i.e., resource elements) is changed according to the multi-antenna configuration. 3GPP R1-074861 suggests, however, that the mapping of the PBCH codeword onto resource elements should not vary with the antenna configuration. According to a second approach, the PBCH codeword is scrambled with different scrambling sequences, wherein the sequence is conditioned on the base station antenna configuration. This approach requires the UE to de-scramble the log-likelihood ratios (LLRs) arising from each hypothesized multi-antenna configuration prior to attempting convolutional decoding and CRC checking. In this second approach, one descrambling operation is required for each antenna configuration hypothesis. A third approach requires changing the Alamouti code (SFBC or SFBC+FSTD) according to the antenna configuration. This would require the UE to support more transmit diversity mapping configurations. Accordingly, some further, low complexity means of assisting the UE in discriminating the antenna configuration is needed.
The various aspects, features and advantages of the disclosure will become more fully apparent to those having ordinary skill in the art upon a careful consideration of the following Detailed Description thereof with the accompanying drawings described below. The drawings may have been simplified for clarity and are not necessarily drawn to scale.